Lead zirconate titanate is a typical piezoelectric material and is used in various piezoelectric devices, such as actuators, oscillators, sensors, and filters. When lead-containing piezoelectric devices are scrapped and exposed to acid rain, however, lead components may leach from the piezoelectric materials into the ground and destroy an ecosystem. Thus, lead-free piezoelectric materials for lead-free piezoelectric devices are being actively studied and developed.
Existing typical widely-studied lead-free piezoelectric materials are piezoelectric materials containing potassium niobate. In the synthesis of potassium-containing piezoelectric materials, however, it is difficult to accurately weigh their raw material (for example, potassium carbonate) powders at an intended molar ratio because of their high hygroscopicity. Furthermore, the piezoelectricity of piezoelectric ceramics containing potassium niobate (KNbO3) sometimes deteriorates over time because piezoelectric materials containing potassium niobate are deliquescent. In addition, the successive phase transition temperature between tetragonal crystals and orthorhombic crystals of piezoelectric materials containing potassium niobate is in an operating temperature range of piezoelectric devices (for example, in the range of 0° C. to 80° C.). Since piezoelectricity varies significantly in the neighborhood of the successive phase transition temperature, the performance of such piezoelectric devices depends greatly on the operating temperature.
NPL 1 described a solid solution (hereinafter referred to as NN-BT) of an antiferroelectric material sodium niobate (NaNbO3) and barium titanate (BaTiO3) as a lead- and potassium-free piezoelectric material. It is disclosed that a piezoelectric ceramic containing sodium niobate and barium titanate at a ratio of 9:1 had a piezoelectric constant d33 of 147 pC/N.
PTL 1 discloses a niobate piezoelectric ceramic containing NN-BT to which cobalt oxide (CoO) is added. The niobate piezoelectric ceramic has a high electromechanical coupling coefficient and high heat resistance. However, some samples were difficult to polarize because of a low resistance as low as 106Ω or less.
PTL 2 discloses a method for manufacturing a niobate piezoelectric ceramic having a high Curie temperature and a satisfactory piezoelectric property. It is disclosed that a niobate piezoelectric ceramic containing a solid solution of NN-BT and strontium titanate (SrTiO3) had a piezoelectric constant d33 in the range of 14 to 126 pm/V.